1. Field of the Invention
This invention relates to viscous fluid coupling devices which serve as a drive for the radiator cooling fan of a vehicle engine, and more particularly, to such coupling devices which control the transmission of an output torque in steps.
2. Description of the Prior Art
In a conventional viscous fluid coupling device which serves as a drive for the radiator cooling fan of a vehicle engine, it is normally necessary to control the communication of the viscous fluid from a reservoir chamber to an operating chamber in response to the temperature of the engine etc. thereby to prevent the engine from excessive cooling and to decrease a loss of horsepower.
In the conventional viscous fluid coupling device, however, a communicating hole for the viscous fluid between the reservoir chamber and the operating chamber is just opened or closed by a thermal responsive valve member upon detection of the prescribed temperature thereby just to control the transmission of the output torque at two stages.
Accordingly, the conventional device results in insufficient or excessive cooling of engine particularly in the region of the temperature for initiating the actuation of the thermal responsive valve member.
As a result, there are drawbacks in that the noise for the acceleration is increased and the loss of the horsepower is also increased.